Hi,
Standard tier has a limit of 5 slots per app while Premium and Isolated tiers has a limit of 20 slots per app. You can see the limits in chart below under staging slots:
App Service limits
There is no extra charge for using slots, however, use is subject to same limits/charges as is true if you only had 1 slot. For example, each slot you use will consume additional storage which counts against the storage limit for the app service plan, each slot will consume CPU/RAM on the instance(s), bandwidth, etc.
-TP